Thin films are important in the manufacture of electronic active devices, such as microelectronics, batteries and fuel cells. In the case of batteries, appropriate thin films can be used to form batteries having low internal series resistance, high capacity and other specific properties required of modern batteries. However, despite continuing improvements in thin film deposition methods, there has been limited success in the manufacture of thin (i.e., <10 μm) homogeneous films. Depending on the film material, available film deposition methods may be expensive, provide an insufficient deposition rate or fail to provide homogeneous films.
A known vacuum method for the production of thin films is laser sputtering. However, this method is generally not suitable for the production of thin films for thickness below approximately 10 μm because of difficulties in achieving homogeneity and structural uniformity for thickness below this value.
Thin films used in batteries are commonly provided by vacuum deposition methods. Other methods, such as mechanical or non-vacuum chemical deposition, can also provide thin films of various types, but these films have certain properties rendering them generally unsuitable for use in batteries or fuel cells. During these deposition process, substantial changes in the chemical or phase structure of the film can occur, resulting in a film having properties which may differ significantly from the initially deposited material. In most cases, these changes are unpredictable and adversely affect resulting film properties.
Different methods for the production of thin films for batteries have been developed. Most efforts have focused on attempts to develop thin, flat, small-area batteries, the batteries having low internal series resistance. Attempts to develop films for batteries having low electronic conductivity and high ionic conductivity have prompted research to develop suitable thin films for use as solid electrolytes and cathodes for high performance batteries, such as Li metal or lithium ion batteries. Solid electrolytes, as well as electrode materials, generally require a high degree of surface purity. In addition, batteries require good electrode/electrolyte contacts.
The above difficulties have been solved in part by a method disclosed by U.S. Pat. No. 6,132,653 to Hunt, et al. Hunt discloses an atmospheric method for chemical vapor deposition (CVD) using a very fine atomization or vaporization of a reagent containing liquid or liquid-like fluid near its supercritical temperature. The resulting atomized or vaporized solution is entered into a flame or a plasma torch, and a powder is formed or a coating is deposited onto a substrate. While the use of the plasma flame described by Hunt can produce nanocrystal powders for several metals and oxides, the method cannot be applied to production of vitreous complex solid electrolytes or to produce layers with a thickness below approximately 10 μm for most applications because the resulting films generally lack uniformity of thickness. Moreover, the disclosed method can result in trapping foreign materials in the film and partial oxidation of the material.
Some current vacuum methods include DC and RF magnetron sputtering, thermal sputtering and molecular beam deposition. The preferred method generally depends on the desired chemical and physical properties of the material, the thickness of the film, and the deposition rate. Using molecular beam techniques, thin dense films from a broad group of materials have been prepared. Films prepared by these procedures have been used in batteries. However, these methods are generally characterized by low deposition rates, the need for ultrahigh vacuum, and difficulty in obtaining RF sputtering targets and for deposition of films having complex compositions.
Regarding batteries utilizing solid electrolytes, minimizing the thickness of solid electrolytes helps reduce the internal series resistance of the battery. However, although films of solid electrolytes having a thickness below 1 μm can be used to produce batteries having low internal resistance, internal short circuits (e.g. from Li dendrites) tend to develop, particularly when the films have large surface areas. In practice films with a minimum thickness of approximately 10–20 μm are used because of technical difficulties in achieving a sufficient structural homogeneity to reduce the battery size. Additionally, expense can be a concern due to the cost of providing certain conventional solid electrolyte materials.
Thus, lack of structural homogeneity can limit the use of available thin film sputtering processes for the production of thin films for batteries. The methods are generally only suitable for production of thin films for superionic conductors having simple stoichiometries. For more complex films, the use of these techniques is generally further limited by a low deposition rate.
Although vacuum thermal sputtering can generally provide up to a 1000 fold increase in deposition rate compared to magnetron and molecular sputtering, thermal sputtering generally produces films having lower density, degraded homogeneity and adhesion to substrates compared to these related methods. The adhesion of the deposited layer to substrates can generally be improved by raising the temperature of the substrate, but can lead to structural changes in the deposited layer, such as increased grain size.
The above limitations of thermal vacuum sputtering result mainly because the process is performed under high vacuum which results in low vapor density and the process uses low energy sputtered particles (below 1 eV). As a result, non-homogenous films are generally formed. Attempts to increase film homogeneity by increasing reactor size, or by increasing the evaporation temperature have been generally unsuccessful, as they lower the deposition efficiency and result in certain system inefficiencies, such as requiring more frequent replacement of the evaporating device.
Accordingly, there is therefore a need for new methods of preparing thin films, particularly films having thicknesses of less than about 50 μl, that are efficiently deposited as homogenous layers substantially free of foreign materials. The deposited layers should provide physical and chemical properties required for efficient function in batteries, fuel cells and other electronic active devices.